Modern electronic devices often have a liquid crystal display to transmit information to the user. In order to make the display readable even in twilight or darkness, the display is generally lit by means of light emitting diodes (LED), but especially in portable devices powered by a battery and/or an accumulator, this also has a shortening effect on the actual operating time of the device. In addition, the requirement for uniform brightness of the display is essential in view of readability, but it increases power consumption to compensate for the loss of light caused by diffuser plates and the like. Instead of using opaque baffles, an alternative is to use a diffractive light pipe structure to conduct the light in the favorable direction, from the light source to the display, whereby there is also more freedom for the disposition of components.
With regard to known techniques related to the art of the invention, reference is made to solutions described in connection with the prior art (FIGS. 1 to 2). A known arrangement is to use ‘thick’, ‘plate-like’ light pipes, on one end of which there is a light source, and on one flat side of the plate with the largest area and/or inside the light pipe there is a lighted object for achieving uniform illumination thereof. It is also known that when the light pipe is made thinner, the distribution of the illumination of the display may become less advantageous. However, there is very little extra room in modern mobile stations and other equipment provided with a display, and thick light pipe structures cannot be used in them without a negative effect on the usability of the device. Thick elements also mean increased material costs to the manufacturer and thereby more pressure on pricing.
There are also known techniques with a thin, plate-like light pipe, from one end of which a light source emits light to the space between the upper and lower surfaces of the light pipe. The bottom of the light pipe may be randomly roughened, e.g. the lower surface of a plate-like light pipe, when the display or a corresponding object to be illuminated is positioned above the upper surface of the light pipe, in the direction of the viewer. The purpose of the roughening is to distribute the light to scatter as uniformly as possible in the direction of the display. There may also be a diffuser, a reflector or a corresponding extra layer under the roughened surface to direct the light that has passed through the roughening back to the light pipe, through it and from it in the direction of the display to increase its illumination.
Although it is the total reflection principle that the propagating light obeys on its way through the light pipe, the random roughening on the light pipe surface may cause problems to the homogeneity of the light, especially at the opposite end to the light source. In other words, much less light comes to the other end of the display than left the first end of the light pipe at the light source. Increasing the number of light sources as well as increasing their power, combined with the use of diffuser plates between the light pipe and the display and/or the light source and the light pipe improve the uniformity of illumination, but also increase power consumption and space requirements.
FIG. 1A illustrates the lighting arrangement of display 1 by using a thin, flat light pipe 3, the lower surface 4 of which is randomly roughened. FIG. 1B represents the local efficiency of the light source, by which light produced by the light source can be converted to backlighting (outcoupling efficiency η hereinafter). The local outcoupling efficiency is represented as a function of location, the coordinate measured from the source end of the light pipe. Because the outcoupling efficiency itself is constant all the way, the brightness of the display as seen by an outside observer is according to FIG. 1C, which thus represents the local brightness of a slice of the display as a function of the distance measured from the end at the light source. FIG. 1D shows in principle how individual rays 5 and 6 leaving a light source L1 propagate in a light pipe 3 and are converted into background light at points 5A to 5E, 6A and 6B.
Another technique for evening out the inhomogeneous brightness, which changes as a function of location as shown in FIG. 1C, is to change the local outcoupling efficiency η as a function of distance by placing dots at which the light is scattered or reflected on the top or bottom of the light pipe. The dots are, for instance, small lenses, which are located at long intermediate distances in the first end of the light source and at shorter intermediate distances in the other end so that there is a smaller difference in brightness B between the first and second end of the display. FIG. 2 illustrates a known arrangement like that described above for illuminating a flat-panel display 7 with a light pipe 9, in which arrangement the lower surface of the light pipe 9 is covered with lenses. The amount of light 8 is greater in the first end of the light pipe 9 near the light source L2 than in the second, opposite end. Because the purpose is to illuminate the display more uniformly, and the local outcoupling efficiency η of the light depends on the local number of scattering and/or reflecting lenses, it is advantageous to make the density of the optical elements smaller near the light source than far from it. To improve the lighting still more, a reflector 10 can be used to return unfavorably directed light back to the direction of the display 7.
Estimating the transmission properties of the light pipe either by experimental or calculatory methods by using known techniques is practically impossible because of the great number of prototypes needed and the number and small size of the lenses, whereby obtaining an acceptable, optimal result with the known technique is questionable.